Due to their light weight, low cost and ease of forming, thermoplastic polymeric materials have been widely used in the fabrication of liquid containers and vessels. More specifically, the prior art has proposed that thermoplastic polymeric materials be used as materials of construction for double-walled containers of the type in which spaced-apart inner and outer wall members enclose a sealed insulation space in which low pressure or vacuum conditions are maintained for enhancement of the insulation quality afforded by the inter-wall space. To maintain the requisite low pressure or vacuum conditions in the insulation space, the prior art has typically employed sorbent materials such as physical adsorbents and getters to take up gases resulting from permeation of gas through the wall members of the insulation space and gas leakage through wall member joints in communication with the insulation space, as well as evolution of gases from the materials of construction of the insulation space.
More recently, it has been found that in double-walled containers of the above-described type, wherein either or both of the inner and outer wall members or portions thereof are formed of thermoplastic polymeric materials, solid physical adsorbent materials can be effectively utilized if such materials are disposed in the insulation space in thermal contact with the wall member forming the container receptacle, when such container is employed for storage and dispensing of cryogenic liquid. In use, the introduction of cryogenic liquid into the container receptacle effects cooling of the wall member bounding the insulation space and the adsorbent material in the insulating space in thermal contact with such wall member, thereby substantially increasing the sorptive affinity of the adsorbent material for gas constituents in the insulation space. In one particularly advantageous arrangement, a thermoplastic polymeric wall member is employed to form the receptacle for cryogenic liquid, with the side of such wall member bounding the insulation space being provided with a layer of pellets of adsorbent, e.g., activated carbon, thereon. In such container construction, the pellets of adsorbent may simply be bonded to the thermoplastic polymeric wall member with an adhesive such as epoxy if the wall member is molded or otherwise formed in a manner which provides a suitable surface for adhesive bonding--i.e., a surface to which the pelleted adsorbent mass will remain adhesively joined, despite cooling of the wall member to cryogenic temperature and subsequent warmup to ambient temperatures. Unfortunately, however, injection molding of the thermoplastic polymeric wall members, such as may desirably be employed, in view of its adaptability to mass production, achievement of close dimensional tolerances on the molded article and production of articles of uniform physical and chemical characteristics, generally produces finished articles having surfaces which are extremely smooth in character and may be unsuitable as substrates for adhesively bonding the adsorbent pellets to the wall surface. This is particularly true where the thermoplastic polymeric material has an intrinsically low surface free energy and is chemically inert so as to be unamenable to chemical etching of the surface, as for example is characteristic of polyethylene and polypropylene.
Accordingly, it is an object of the present invention to provide a method of treating a thermoplastic polymeric surface to enhance the adhesive bondability of same.
It is another object of the present invention to provide an improved method of forming a particulate coating on a thermoplastic polymeric surface.
It is still another object of the invention to provide a thermoplastic polymeric article to which pellets of adsorbent material are strongly adhesively bonded.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.